1. Field of the Invention
The present invention relates generally to glass-ceramics, in particular to substantially transparent glass-ceramics containing a microstructure comprising nanocrystalline hexagonal ZnO crystals as the major crystalline phase.
2. Technical Background
Glass-ceramics are polycrystalline materials formed by a controlled crystallization of a precursor glass. In general, the method for producing such glass-ceramics customarily involves three fundamental steps: first, melting a glass-forming batch containing the selected metallic oxides; second, cooling the melt to a temperature at least below its transformation range, while simultaneously forming a glass body of a desired geometry; and third, heating the glass body to a temperature above the transformation range of the glass in a controlled manner to generate crystals in situ. To develop nuclei in the glass, the glass will be heated initially to a temperature within or somewhat above the transformation range for a period of time; although there are certain compositions that are known to be self-nucleating and thus do not require the development of nuclei. Thereafter, the temperature will be raised to temperatures where crystals can grow from the nuclei. The resulting crystals are typically uniformly distributed and fine-grained. Internal nucleation permits glass-ceramics to have favorable qualities such as a very narrow distribution of particle size and a highly uniform dispersion of crystals throughout the glass host.
Transparent glass-ceramics are known in the art, with the classic study relating to transparency being authored by G. H. Beall and D. A. Duke in “Transparent Glass Ceramics,” Journal of Material Science, 4, pp. 340-352 (1969). Glass-ceramic bodies will display transparency to the human eye when the crystals present therein are considerably smaller than the wavelength of visible light. In other words, transparency typically results from crystals less than 50 nm—preferably as low as 10 nm—in size, if there is a major refractive index difference between crystal and glass. Transparency in glass-ceramics, alternatively, can also be produced with crystals larger than 50 nm if the crystal birefringence and the index of refraction mismatch between the crystal phase and the glassy phase are both low. Transparent glass-ceramics, doped with transition elements can combine the optical efficiency of crystals with the flexibility of the forming of glass. For example, both bulk (planar substrates) and fiber forms can be fabricated from these glass-ceramics.
Recently, researchers have concentrated much effort to develop transparent glass-ceramics as hosts for transition metal ions. Transition metals have been used as optically active dopants in crystalline hosts because they fluoresce in the near infrared (700 nm to 2000 nm) region. Given the useful wavelength range and relatively wide bandwidth of many transition-metal dopants, much interest has arisen for their use in optical telecommunication applications, with the region from 1000 nm to 1500 nm being of particular interest. The current optical telecommunication medium is glass-based optical fiber. Inclusion of transition metal dopants into glasses, however, has unfortunately not produced fluorescence performances as good as in crystalline materials. The performance of transition metal ions tends to degrade in amorphous hosts, where the crystal field strength is much smaller than in even crystalline hosts.
Suitable glass-ceramic hosts, therefore, must be tailored such that transition elements will preferentially partition into the crystal phase. Some of these glass-ceramics have come from compositions such as those discussed the following applications. Co-pending U.S. patent application, Pub. No. 2002/0028739, entitled FORSTERITE GLASS-CERAMICS OF HIGH CRYSTALLINITY AND CHROME CONTENT, by George H. Beall, et al., and co-pending U.S. Pat. No. 6,300,262, entitled TRANSPARENT FORSTERITE GLASS-CERAMICS, by George H. Beall both of which disclose a family of, and a method of making, glass compositions based in the K2 O—MgO—Al2O3—SiO2 system. U.S. Pat. No. 6,297,179, entitled TRANSITION-METAL GLASS-CERAMIC GAIN MEDIA, by George H. Beall et al., discloses transition-metal-doped glass-ceramic materials used as gain media or pump laser fiber in optical amplifiers and lasing mechanisms. WO 01/28944 entitled TRANSPARENT LITHIUM ORTHOSILICATE GLASS-CERAMICS, by George Beall, et al., discloses a family of glass compositions within the ternary Mg2SiO4—Zn2SiO4—Li4SiO4 system and exhibiting a predominate orthosilicate crystal phase. Lastly, U.S. Pat. No. 6,303,527 entitled Transparent Glass-ceramics Based on Alpha- and Beta-Willemite, by L. R. Pinckney discloses substantially and desirably totally transparent glass-ceramics, and which contain a willemite predominant crystal phase within the ternary Mg2SiO4—Zn2SiO4—Li4SiO4 system. Each of these patents and applications are co-assigned to the present assignee and the entire contents of both of these applications are incorporated herein by reference.
Transparent glass-ceramics which contain relatively small numbers of crystals can be of great use in cases where the parent glass provides an easy-to-melt or an-easy-to-form vehicle for a crystal. The single crystals may be difficult or expensive to synthesize, however they provide highly desirable features, such as optical activity. The crystals in the glass-ceramic are generally oriented randomly throughout the bulk of the glass contrary to a single crystal which has a specific orientation. Random orientation, and consequent isotropy, are advantageous for many applications. One example is that of optical amplifiers, where polarization-independent gain is imperative.
Bulk and thin-film ZnO materials are well known in the art. In general, ZnO is a wide band gap (3.3 eV) semiconductor material. One application known in the art is ZnO varistors which are ceramic composites used as voltage stabilization and transient surge suppression in electric power systems. The key feature of ZnO varistors are their high nonlinearity of the current-voltage characteristics. Also known in the art are transparent, electrically conductive polycrystalline films based on doped ZnO. Doped ZnO is an n-type semiconductor and is one of a family of transparent conducting oxides (TCOs) that are used in energy conserving windows, oven windows, “smart” windows and front-surface electrodes for solar cells and flat panel displays. Recently, a planar waveguide device for 1.55 μm amplification based on erbium-doped nanocrystalline ZnO was demonstrated. Lastly, prior art reveals the preparation of ZnO as nanoparticles in numerous colloidal solutions by various methods, including sol-gel, laser vaporization-controlled condensation, reversed micelle techniques. These particles exhibit quantum size effects; their band gap absorption and emission are blue-shifted with respect to bulk ZnO and their visible emission, in the 500 nm region, shows wavelength and lifetime dependence on size.
Although it is known in the art to utilize transparent ZnO, both in bulk form and as thin films, in optical applications, nothing has been found in the prior art to suggest the formation of transparent ZnO-crystal containing glass-ceramics capable of being utilized in both optical and dielectric applications.
Accordingly, the primary object of the present invention is to provide nanocrystalline hexagonal ZnO glass-ceramics glass-ceramic material which are substantially and desirably totally transparent which are capable of being doped with ingredients which confer useful optical and dielectric properties including, high absorption in the near infrared and microwave wavelengths.
Other objects and advantages of the present invention will be apparent from the following description.